Hello,
I have dealt with this issue since the day i bought this car.
The previous owner kept it pristine, however there was a musty smell that had accompanied the purchase and and after taking new ownership i pulled the back seat up where there was this clear slime that resembled what was once water laying on top of the neoprene seat pan insulators.

I thoroughly cleaned beyond bare knuckles, draped off the cabin interior in plastic sheeting and grit blasted the floor pan then sprayed metal etching primer down as you see in the photo.
The car has been in a tent for ten years, no rain, what you are seeing is the result from simple washing dust off the car every spring.

(The $63K pop rivets and steel pan plugs and plastic mastic in the floor pans are an altogether other liquid leaking story of their own demonic entity, to be tackled and exorcised later on.)

Water runs down hill, but it can also wick itself uphill.

I cannot for the life of me understand where this is all coming from.

Even before the repairs or the tent i would come out in spring and find water sitting in the seat pans from just being rain soaked sitting on the driveway, and with a cheap car cover on it over winter.
All i could figure is it ran down from some area then somehow wicked its way up through the body seams of the seat pan.
( Ignore the access holes, the plugs and seam sealer were removed to drain and wash clean the pan bottoms)
Where you see the area circled, i took a formed piece of sheet metal and soldered those holes shut to no avail, it is still coming from someplace and its not from the fuel cell area or rear glass or rear deck area.

Its wicking from beneath somewhere. Perhaps some pinholes? But not to yield the amount of water that has been found!



I even used to keep a small milk house heater inside the cabin on a timer when the leather was in it that would run an hour in the morning and an hour at night to keep it dry inside, simulating daily use.

Cheers for the Help in advance,
Stür

 

Even though you say you've checked it, I'd still put money on the rear window seal to body area.

One way that is quite good to check these things (although maybe a bit more difficult with a white car) is to leave out the rear seats, dry everything thoroughly and sprinkle talcuum powder in the seat pan and as far up the interior body sides as possible. It then becomes easier to see the moisture path after it leaks.

Good luck

Paul